The present invention relates to servo control systems, and in particular to a servo control system for making a focus point of a laser beam keep track of a plane of a recording disc on which information is recorded. More specifically, the invention is concerned with a novel servo acquisition system whereby the focus point of the beam is moved toward the disc in an open loop mode until the recording plane is detected, whereupon closed loop operation is initiated.
In disc recording systems in which information is recorded in the form of a series of pits along spiral or concentric tracks by the impingement of a laser beam on a light sensitive material, such material is usually sandwiched between two protective layers. The beam impinging side of the protective layers is formed of a transparent material to allow the beam to be focused precisely on the recording plane. Because of reflections from the upper surface of the protective layer, the focus point of the beam would be servo-controlled to the upper surface plane rather than on the recording plane if no appropriate provision is made. Because of the small beam spot size which is usually on the order of 1 micrometer compared to the spacing between the upper surface and the recording plane, the beam focusing on the upper surface is detrimental both to information recording and reading.
In one prior art system, a derived error signal is biased to a DC level sufficient to move the focus point toward the record disc. The speed of the focus point then increases as a result of the error signal which increases in level due to reflections from the disc planes. A speed detector is provided to detect when the focus moving speed reaches a predetermined value which corresponds to the level of reflections from the recording plane. A braking mechanism is provided to initiate application of a brake in response to the moving speed reaching the predetermined value to prevent the focusing lens from hitting the disc surface. The brake is applied until the focus point reaches the recording plane, whereupon the servomechanism comes into play to allow the focus point to keep track of the recording disc which may fluctuate vertically as it revolves with the turntable. Since the braking mechanism is of a precision type, the drive mechanism becomes complicated, resulting in a bulky structure.